It means that God exists as a pure and perfect spirit being, not having size or dimensions. Thus, the very nature of his existence is the ultimate existence, far superior to and totally different from anything he has created.

Emery Bancroft defines God’s Spirit nature more precisely: God as Spirit is incorporeal, invisible, without material substance, without physical parts or passions and therefore free from all temporal limitations. (Elemental Theology, p. 23) Some have been disturbed as they compare these statements with certain Old Testament expressions that speak of God’s arms (Deut 33:27), his eyes (Ps 33:18), his ears (2 Kgs 19:16), and his mouth (Isa 58:14). However, these terms are simply anthropomorphic expressions, terms used to explain some function or characteristic of God by using words descriptive of human elements. Robert Lightner connects God’s Spirit nature to these Old Testament descriptions:

Such expressions do not mean that God possesses these physical parts. He is Spirit (John 4:24). Rather, they mean since God is Spirit and eternal, He is capable of doing precisely the functions which are performed by these physical properties in man. (The God of the Bible, p. 67)

It does not imply God is made of pure thought or energy, or of vapor, steam, space or air. It does not imply God consists of some exotic and unknown refined or radically altered form of either matter or antimatter.